Plant derived nutrition always contains secondary plant metabolites next to the classical macronutrients (carbohydrates, proteins and fat). Within the plant these substances have essential protective effects, e.g., they provide protection against UV-irradiation or attacks by insects, fungi or bacteria. Approximately 5-10,000 different compounds are regularly supplied with the diet.

Whereas in the past the focus of attention was mainly of plant toxins – i.e., constituents with highly potent pharmacologic effects – the bulk of non-toxic compounds such as the flavonoids which are ubiquitous in plants have usually been neglected as “non-nutrients”. Today there is accumulating evidence that these secondary plant metabolites participate in the fine-tuning of many physiological mechanisms in a vitamin-like manner. Even more, the human organism seems to count on their presence.

Secondary plant metabolites display many health-promoting effects in the human organism, e.g., in the digestive tract, the cardiovascular system, the control of blood lipids and glucose turnover, and especially in the immune system. This opens new perspectives for dietary prevention of immunologic processes such as inflammation, but also cancer development. For both aspects the advantages of a sufficient dietary supply with selected secondary plant metabolites have been amply examined and confirmed.

Frequently certain biomarkers are held responsible for the effects. The effect of plant extracts is, however, generally superior over the sum of their “active ingredients”. In the extracts synergistic effects of different active constituents may be expected. Typically the secondary plant metabolites simultaneously act on multiple targets. The effect strength may be rather small at every single target enzyme or receptor, the accumulated single effects do, however, typically reach a total effect of approximately 10 to 20 percent above what can be reached with placebo – which is an overall positive effect clearly recognizable by the patient.

Well-examined examples of secondary plant metabolites with an effect in the immune system are extracts from green tea (with the lead marker epigallocatechingallate EGCG), broccoli (isothiocyanates of the sulphoraphan type), pomegranate (polyphenols), grapes (resveratrol), Curcuma (curcumin) or ginger (gingeroles and shoagoles). For all these substances mechanisms related to inflammation and tumour prevention have been demonstrated. Typical immunologic mechanisms involve, e.g., the inhibition of mediators such as TNFα or NF-κB, the inhibition of matrixmetalloproteases or of the complement system, or the normalization of the rate of apoptosis, which is usually found abnormal in inflammation and carcinogenesis.

Overshooting reactions are usually not to be expected. Especially when secondary plant metabolites are not sufficiently supplied through the diet – which is regularly the case in severe diseases, the specific supply with certain plant extracts in doses corresponding to usual dietary quantities may result in an improved function of the immune system, whereas no negative consequences are to be expected in healthy tissues.